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Forms of Laser Propulsion
There are several forms of laser propulsion. Ablative laser propulsion Ablative Laser Propulsion (ALP) is a form of beam-powered propulsion in which an external pulsed laser is used to burn off a plasma plume from a solid metal propellant, thus producing thrust. The measured specific impulse of small ALP setups is very high at about 5000 s (49 kN·s/kg), and unlike the lightcraft developed by Leik Myrabo which uses air as the propellant, ALP can be used in space. Material is directly removed from a solid or liquid surface at high velocities by laser ablation by a pulsed laser. Depending on the laser flux and pulse duration, the material can be simply heated and evaporated, or converted to plasma. Ablative propulsion will work in air or vacuum. Specific impulse values from 200 seconds to several thousand seconds are possible by choosing the propellant and laser pulse characteristics. Variations of ablative propulsion include double-pulse propulsion in which one laser pulse ablates material and a second laser pulse further heats the ablated gas, laser micropropulsion in which a small laser on board a spacecraft ablates very small amounts of propellant for attitude control or maneuvering, and space debris removal, in which the laser ablates material from debris particles in low Earth orbit, changing their orbits and causing them to reenter. University of Alabama Huntsville Propulsion Research Center has researched ALP. Pulsed plasma propulsion A high energy pulse focused in a gas or on a solid surface surrounded by gas produces breakdown of the gas (usually air). This causes an expanding shock wave which absorbs laser energy at the shock front (a laser sustained detonation wave or LSD wave); expansion of the hot plasma behind the shock front during and after the pulse transmits momentum to the craft. Pulsed plasma propulsion using air as the working fluid is the simplest form of air-breathing laser propulsion. The record-breaking Lightcraft, developed by Leik Myrabo of RPI (Rensselaer Polytechnic Institute) and Frank Mead, works on this principle. Another concept of pulsed plasma propulsion is being investigated by Prof. Hideyuki Horisawa. CW plasma propulsion A continuous laser beam focused in a flowing stream of gas creates a stable laser sustained plasma which heats the gas; the hot gas is then expanded through a conventional nozzle to produce thrust. Because the plasma does not touch the walls of the engine, very high gas temperatures are possible, as in gas core nuclear thermal propulsion. However, to achieve high specific impulse, the propellant must have low molecular weight; hydrogen is usually assumed for actual use, at specific impulses around 1000 seconds. CW plasma propulsion has the disadvantage that the laser beam must be precisely focused into the absorption chamber, either through a window or by using a specially-shaped nozzle. CW plasma thruster experiments were performed in the 1970s and 1980s, primarily by Dr. Dennis Keefer of UTSI and Prof. Herman Krier of the University of Illinois at Urbana-Champaign. Heat Exchanger (HX) Thruster The laser beam heats a solid heat exchanger, which in turn heats an inert liquid propellant, converting it to hot gas which is exhausted through a conventional nozzle. This is similar in principle to nuclear thermal and solar thermal propulsion. Using a large flat heat exchanger allows the laser beam to shine directly on the heat exchanger without focusing optics on the vehicle. The HX thruster has the advantage of working equally well with any laser wavelength and both CW and pulsed lasers, and of having an efficiency approaching 100%. The HX thruster is limited by the heat exchanger material and by radiative losses to relatively low gas temperatures, typically 1000 - 2000 C, but with hydrogen propellant, that provides sufficient specific impulse (600 – 800 seconds) to allow single stage vehicles to reach low Earth orbit. The HX laser thruster concept was developed by Jordin Kare in 1991; a similar microwave thermal propulsion concept was developed independently by Kevin L. Parkin at Caltech in 2001. A variation on this concept was proposed by Prof. John Sinko and Dr. Clifford Schlecht as a redundant safety concept for assets on orbit. Packets of enclosed propellants are attached to the outside of a space suit, and exhaust channels run from each packet to the far side of the astronaut or tool. A laser beam from a space station or shuttle vaporizes the propellant inside the packs. Exhaust is directed behind the astronaut or tool, pulling the target towards the laser source. To brake the approach, a second wavelength is used to ablate the exterior of the propellant packets on the near side. Laser electric propulsion A general class of propulsion techniques in which the laser beam power is converted to electricity, which then powers some type of electric propulsion thruster. A small quadcopter has flown for 12 hours charged by a 2.5 kW laser, using 170 watt photovoltaic arrays as the power receiver, and a laser has been demonstrated to charge the batteries of an unmanned aerial vehicle in flight for 48 hours. For spacecraft, laser electric propulsion is considered as a competitor to solar electric or nuclear electric propulsion for low-thrust propulsion in space. However, Leik Myrabo has proposed high-thrust laser electric propulsion, using magnetohydrodynamics to convert laser energy to electricity and to electrically accelerate air around a vehicle for thrust. Photonic Laser Thruster (PLT) Photonic Laser Thruster (PLT) is a pure photon laser thruster that amplifies photon radiation pressure by orders of magnitude by exploiting an active resonant optical cavity formed between two mirrors on nearby paired spacecraft. PLT is predicted to be able to provide the thrust to power ratio (a measure of how efficient a thruster is in terms of converting power to thrust) approaching that of conventional thrusters, such as laser ablation thrusters and electrical thrusters. In December 2006, Dr. Young K. Bae successfully demonstrated the photon thrust amplification in PLT for the first time with an amplification factor of 3,000 under NASA sponsorship (NIAC).12 Scaling-up of PLT is highly promising, and PLT is predicted to enable wide ranges of next generation space endeavors. Low thrust (milli-Newton) PLTs enable nanometer precision spacecraft formation, for example Photon Tether Formation Flight (PTFF), for forming ultralarge space telescopes and radars. A significant limitation of this technique is that light must bounce with nearly no loss between the two mirrors on the paired satellites. Diffraction effectively rules this technique out for mirrors not much closer than the distance at which the mirror's Airy disk is equal to the size of the other mirror: around 150 km for a 1 m diameter mirror, scaling linearly with larger diameters. The Photonic Laser Thruster offers continuous and constant thrust. This feature offers constant acceleration to the spacecraft. However, the spacecraft is still under the influence of the Sun's gravity during interplanetary traveling. In such case, the spacecraft's trajectory cannot be a straight line and traveling time may not be simply estimated. Since 2011, Dr. Fu-Yuen Hsiao in Tamkang University has been investigating the trajectories of spacecraft with PLT under the two-body problem and three-body problem assumptions. Zero-velocity contours, trajectory evolution and trajectory design are investigated in Hsiao's work.